Group based PDCCH capability for LTE

ABSTRACT

A group-based PDCCH capability in LTE. Information common to a group of UEs, such as those common to a virtual carrier, may be signalled in a group search space within the PDCCH. This common information may include the location of a further control region embedded in the virtual carrier which contains UE-specific information for access the resources of the virtual carrier. Additional methods assign UEs a group identity by implicit signalling, and determine the aggregation level of the group-based PDCCH search space.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is based on PCT/GB2013/053191 filed Dec. 2, 2013and claims priority to British Patent Application 1221717.0, filed inthe UK IPO on 3 Dec. 2012 and British patent application 1221729.5,filed 3 Dec. 2012, the entire contents of each of which beingincorporated herein by reference.

BACKGROUND OF THE INVENTION

The present invention relates to methods, systems and apparatus fortransmitting data to and/or receiving data from mobile terminals in awireless communications system.

Embodiments of the present invention can for example allocatetransmission resources for, and transmit control data to, groups ofmachine type communication (MTC) devices in cellular telecommunicationsnetworks having orthogonal frequency division multiplex (OFDM) basedradio access technology (such as WiMAX and LTE).

Certain classes of telecommunications device, such as MTC devices (e.g.semi-autonomous or autonomous wireless communication terminals), support“low capability” communication applications that are characterised, forinstance, by the transmission of small amounts of data at relativelyinfrequent intervals. MTC devices are constructed so that individuallythey represent little burden on telecommunications networks and thus canbe deployed in greater numbers than equivalent “full capability”terminals in the same networks.

In many scenarios, it is preferable to provide terminals dedicated tosuch “low capability” communication applications with a simple receiverunit (or transceiver unit) having capabilities more commensurate withthe amount of data likely to be transmitted to (or from) the terminal.This more limited capability contrasts with the capabilities of theconventional mobile telecommunications terminals, such as smartphones,which share access to the same telecommunications networks.

To support MTC terminals, it has been proposed to introduce a “virtualcarrier” operating within a bandwidth of one or more “host carriers”:the proposed virtual carrier concept preferably integrates within thetransmission resources of conventional OFDM based radio accesstechnologies and subdivides frequency spectrum in a similar manner toOFDM. Unlike data transmitted on a conventional OFDM type downlinkcarrier, data transmitted on the virtual carrier can be received anddecoded without needing to process the full bandwidth of the downlinkOFDM host carrier. Accordingly, data transmitted on the virtual carriercan be received and decoded using a reduced complexity receiver unit:with concomitant benefits such as reduced complexity, increasedreliability, reduced form-factor and lower manufacturing cost.

The virtual carrier concept is described in a number of co-pendingpatent applications (including GB 1101970.0 [2], GB 1101981.7 [3], GB1101966.8 [4], GB 1101983.3 [5], GB 1101853.8 [6], GB 1101982.5 [7], GB1101980.9 [8] and GB 1101972.6 [9]), the contents of which areincorporated herein by reference.

In one implementation of the virtual carrier (VC) concept, described inco-pending patent application GB 1121767.6 [11], VC capable MTC devicesare presumed to receive only certain OFDM symbols across all hostcarrier (HC) sub-carriers (the HC control region)—the remaining OFDMsymbols are typically received across one of a plurality of VC bandwidthranges. The VC provides dedicated VC control regions amongst the symbolsreceived across the VC bandwidth range.

In conventional LTE, at least some of the resource elements (REs)comprising this HC control region are defined by specification to form anumber of so-called control channel elements (CCEs). A physical downlinkcontrol channel (PDCCH), for providing control information to devices,comprises a number of CCEs. The number of CCEs comprising a particularPDCCH depends on the aggregation level determined by the eNodeB (seelater for discussion of aggregation levels). A UE must search throughsome number of the CCEs in the control region to determine if there areany that comprise PDCCHs containing control information pertinent to theUE. Some CCEs are searched by all UEs, these CCEs comprising a so-calledcommon search space (CSS), and some CCEs are not searched by all UEs,these CCEs comprising a so-called UE-specific search space (UESS). A CCEmay be part of more than one search space. Typically, PDCCHs comprisingCCEs in the common search space contain information relevant to all UEsin a cell and PDCCHs comprising CCEs in a UE-specific search spacecontain information relevant only to one UE.

The HC control region has a limited number of REs and this limitationmay restrict the number of MTC devices that may be deployed: since eachMTC device requires a corresponding UESS. It is predicted that thenumber of MTC devices will increase markedly in the coming years and thelimitation on REs in LTE can be expected to restrict many MTC scenarios.

An efficient operation of a wireless telecommunications system for MTCdevices is therefore desirable.

SUMMARY OF THE INVENTION

According to a first aspect of the invention there is provided awireless communications system for transmitting data to and/or receivingdata from mobile terminals, the wireless communications systemcomprising:

one or more base stations, each of which includes a transmitter and areceiver configured to provide a wireless access interface forcommunicating data to and/or from the mobile terminals, the wirelessaccess interface providing a plurality of communications resourceelements across a first frequency range,

wherein the wireless access interface provided by the one or more basestations includes a plurality of time divided sub-frames, and at leastone of the sub-frames includes:

a first control region in a first part of the sub-frame forcommunicating first signalling information to one or more of the mobileterminals, the first control region including a plurality of controlchannel resource elements, a first subset of said control channelresource elements providing a group control channel, the group controlchannel being associated with a group identifier and at least partiallyencoded using the group identifier; and

a second control region in a second part of the sub-frame, distinct fromthe first part of the sub-frame occupied by the first control region,the second control region being for communicating second signallinginformation to a predetermined group of the mobile terminals,

and wherein the group control channel contains information indicative ofthe location of the second control region, said information beingaccessible by applying the group identifier to the group control channeland each of the predetermined group of mobile terminals using the samegroup identifier.

The wireless communications system therefore implements a group-basedcontrol channel capability (the group search space) which transmitsinformation common to a group of UEs, such as those common to a virtualcarrier. This includes in particular the location of a further controlregion which may be embedded in the virtual carrier which containsUE-specific information for providing access to the resources of thevirtual carrier.

The potential capacity problem in the host carrier control region (i.e.the physical downlink control channel (PDCCH)) in an MTC scenarioidentified above is thus addressed by allowing information common to agroup of UEs, but not intended for broadcast to all UEs, to be signalledefficiently on PDCCH in a new group-specific search space (GSS). Thismakes more efficient use of PDCCH capacity without imposing anunnecessary processing load on non-VC UEs.

Various further aspects and embodiments of the invention, includingmechanisms for assigning a group identity to UEs by implicit signallingand for determining the aggregation level of a group-based PDCCH searchspace, are provided in the accompanying independent and dependentclaims.

It will be appreciated that features and aspects of the inventiondescribed above in relation to the first and other aspects of theinvention are equally applicable to, and may be combined with,embodiments of the invention according to the different aspects of theinvention as appropriate, and not just in the specific combinationsdescribed above. Furthermore features of the dependent claims may becombined with features of the independent claims in combinations otherthan those explicitly set out in the claims.

BRIEF DESCRIPTION OF DRAWINGS

Embodiments of the present invention will now be described by way ofexample only with reference to the accompanying drawings where likeparts are provided with corresponding reference numerals and in which:

FIG. 1 provides a schematic diagram illustrating an example of aconventional mobile telecommunication network;

FIG. 2 provides a schematic diagram illustrating a conventional LTEradio frame;

FIG. 3 provides a schematic diagram illustrating an example of aconventional LTE downlink radio sub-frame;

FIG. 4 provides a schematic diagram illustrating an example of a LTEdownlink radio sub-frame in which a narrow band virtual carrier has beeninserted at the centre frequency of the host carrier, the virtualcarrier region abuts the wideband PDCCH control region of the hostcarrier—making a characteristic “T-shape”;

FIG. 5 provides a schematic diagram illustrating an example of a LTEdownlink radio sub-frame in which virtual carriers have been inserted ata number of frequencies of the host carrier;

FIG. 6A provides a schematic diagram illustrating an example of a LTEdownlink radio sub-frame in which the HC control region is supplementedby a VC control region (a VC PDCCH region) within the restrictedfrequency band of a virtual carrier;

FIG. 6B provides a schematic diagram illustrating an example of a LTEdownlink radio sub-frame in which the HC control region is supplementedby a VC control region (a VC PDCCH region) and a VC-EPDCCH within therestricted frequency band of a virtual carrier and an EPDCCH controlregion;

FIG. 6C provides a schematic diagram illustrating the relationshipbetween CCEs and REs within the HC control region;

FIG. 7 provides a schematic illustration of schemes for accessingcontrol channel elements within the GSS and CSS;

FIG. 8A provides a schematic illustration of contention-based randomaccess procedure in LTE;

FIG. 8B shows the structure of a conventional MAC RAR message;

FIG. 8C shows the structure of a MAC RAR message extended to include aG-C-RNTI;

FIG. 9 provides a schematic diagram illustrating a part of an LTEcellular telecommunications network adapted to provide radio access toconventional LTE terminal and reduced capacity terminals in accordancewith an embodiment of the present invention; and

FIG. 10 illustrates the operation of a mobile terminal in accordancewith an embodiment of the invention.

DETAILED DESCRIPTION

Previous co-pending applications have discussed in detail the design andoperation of some parts of a so-called virtual carrier (VC), embedded ina classical host carrier (HC), suitable for use particularly in LTEnetworks serving machine-type communication (MTC) devices among theirmix of user equipment terminals (UEs). One particular version of the VCdesign is a so-called ‘T-shaped’ VC, a fuller description of which maybe found in co-pending patent application number GB 1121767.6 [11]. Astructure for this is illustrated in FIG. 4. In such a VC, the VC-UE isassumed to be able to decode the wideband control region on the HC, butis thereafter confined to relatively narrowband resources for physicaldownlink shared channels (PDSCH), etc. on the VC.

The control region defined in current releases of LTE includes thePCFICH, PHICH, PDCCH and reference signals (RS). Of interest here is thephysical downlink control channel (PDCCH). A UE must search through thecontrol region to find two sets of information carried on PDCCH: a firstset that is broadcast to all UEs, and a second set that is intended forthe UE alone. This searching is done by “blindly decoding” all possiblelocations and combinations of resource elements (REs) that could formthe UE's PDCCH, and the channel specifications define how the REs shouldbe combined into PDCCH candidates.

The procedure for searching all possible PDCCH candidates is termed“blind decoding” as no information is provided in advance that wouldallow a more targeted search.

This means that all UEs scheduled in one subframe must have theirrespective control information embedded in the control region. With alarge number of UEs, such as may arise in MTC scenarios, there could bethe possibility of insufficient resource available to assign eachscheduled UE a PDCCH in the control region of one subframe.

In LTE, the identifier used to direct data to any given UE is known as aRadio Network Temporary Identifier. Depending upon the context within acommunication session, the RNTI may take one of a number of forms. Thusdata that is UE specific uses either a C-RNTI (cell RNTI) or a temporaryC-RNTI; data intended for broadcast of system information uses a SI-RNTI(system information RNTI); paging signals use a P-RNTI (paging RNTI);messages concerning the random access procedure (RA procedure) useRA-RNTI (random access RNTI), etc. The C-RNTI thus uniquely identifies aUE in a cell. RNTIs are assigned from a range of 16-bit values, andspecifications restrict which RNTIs may be taken from which rangeswithin the total possible range. Some values are not permitted for useas any RNTI, referred to in this description as ‘reserved RNTIs’. Incurrent versions of specifications, these are the range FFF4 to FFFCinclusive, in hexadecimal notation.

A UE determines whether a particular PDCCH within the control region isintended for itself by attempting to decode each possible set of REsthat could be a PDCCH, according to the specifications and the eNBconfiguration. In LTE, each RRC-connected UE is assigned a 16-bitC-RNTI, which allows a maximum of about 65000 users to be RRC connected.The assigned C-RNTI (or other UE ID) is used to uniquely address controlinformation to specific UEs in the cell. To reduce signalling overhead,the UE ID will not be sent explicitly. Instead, part of the PDCCH dataintended for the UE is scrambled (masked) with a mask uniquelyassociated with the UE ID by the eNodeB (or other network accessentity). In a particular example, the CRC bits (cyclic redundancychecking bits—primarily used in error correction procedures) arescrambled using the C-RNTI.

PDCCH data scrambled with the UE's own C-RNTI may only be de-scrambledwith that same C-RNTI. Thus, in the example, each UE descrambles thereceived CRC bits with its own mask before doing a CRC check.

C-RNTIs are assigned to UEs by the network during the random access (RA)procedure. A similar process is conducted to locate any broadcastinformation, which has CRC scrambled by a common RNTI known to all UEsin the cell, such as the P-RNTI or the SI-RNTI.

In the absence of a separate UE identifier, 2G and 3G technologies seekto identify UEs by reference to the International Mobile SubscriberIdentity (IMSI)—strictly, the IMSI is a subscriber identifier oftenassociated with subscriber identification module cards (“SIMs”). TheIMSI is still a feature of LTE technologies and, where a single SIM ispresent in each UE, the IMSI may be used as a further identifier of theUE within the cell.

Control information is packaged for transmission over the PDCCH instandardised Downlink Control Information (DCI) messages—these DCImessages take different formats depending upon their purpose. DCIformats include: uplink grant signals; downlink shared channel resourceallocation signals; Transmit Power Control (TPC) commands, which adaptthe transmit power of the UE to save power; and MIMO precodinginformation. A more detailed discussion of 3 GPP standard DCI formatsmay be found in 3 GPP TS 36.212 (Section 5.3.3.1) which is incorporatedherein by reference. In a “T-shaped” VC as discussed above, there couldbe insufficient capacity in the control region to provide the manyPDCCHs that may be needed in an MTC scenario. Furthermore, to providecontrol information specific to the VC, a straightforward solution wouldbe to create a new DCI format. However, this step in isolation wouldincrease the blind decoding load for UEs aware of the new format and,since MTC devices should be low power and low cost, this could be anundesirable approach.

UEs configured to use the VC will typically share a certain amount ofcommon control information relating the operation of the VC. Thisinformation is not relevant to all UEs in the cell, so is not suitablefor transmission in the common search space (CSS) of PDCCH: nor howeverneed it be transmitted on a multitude of PDCCHs to many UEs in theirrespective UE-specific search spaces (UESS). The latter approach isdeprecated as this is one possible cause of control region overload.

If common VC control information were transmitted in the CSS using sucha new DCI format, then it would mean all UEs would be forced to attemptto “blind decode” PDCCHs containing another DCI format; even legacy UEs,for which the format has no meaning and the VC control information, novalue. Furthermore, it could be difficult to carry control signallingfor multiple independent VCs within an HC all within the CSS.

Therefore, solutions that mitigate the potential limitations on PDCCHcapacity and achieve this with minimal incremental blind-decoding loadfor UEs are of significant interest. FIG. 1 provides a schematic diagramillustrating some basic functionality of a conventional mobiletelecommunications network, using for example a 3 GPP defined UMTSand/or Long Term Evolution (LTE) architecture.

The network includes a plurality of base stations 101 connected to acore network 102. Each base station provides a coverage area 103 (i.e. acell) within which data can be communicated to and from terminal devices(also referred to as mobile terminals, MT or User equipment, UE) 104.Data is transmitted from base stations 101 to terminal devices 104within their respective coverage areas 103 via a radio downlink. Data istransmitted from terminal devices 104 to the base stations 101 via aradio uplink. The core network 102 routes data to and from the terminaldevices 104 via the respective base stations 101 and provides functionssuch as authentication, mobility management, charging and so on.

Mobile telecommunications systems such as those arranged in accordancewith the 3 GPP defined Long Term Evolution (LTE) architecture use anorthogonal frequency division multiplex (OFDM) based interface for theradio downlink (so-called OFDMA) and the radio uplink (so-calledSC-FDMA).

FIG. 2 shows a schematic diagram illustrating an OFDM based LTE downlinkradio frame 201. The LTE downlink radio frame is transmitted from an LTEbase station (known as an enhanced Node B) and lasts 10 ms. The downlinkradio frame comprises ten sub-frames, each sub-frame lasting 1 ms. Aprimary synchronisation signal (PSS) and a secondary synchronisationsignal (SSS) are transmitted in the first and sixth sub-frames of theLTE radio frame, in frequency division duplex (FDD). A physicalbroadcast channel (PBCH) is transmitted in the first sub-frame of theLTE radio frame. The PSS, SSS and PBCH are discussed in more detailbelow.

FIG. 3 is a schematic diagram of a grid which illustrates the structureof an example conventional downlink LTE sub-frame. The sub-framecomprises a predetermined number of “symbols”, which are eachtransmitted over a respective 1/14 ms period. Each symbol comprises apredetermined number of orthogonal sub-carriers distributed across thebandwidth of the downlink radio carrier. Here, the horizontal axisrepresents time while the vertical represents frequency.

The example sub-frame shown in FIG. 3 comprises 14 symbols and 1200sub-carriers spread across a 20 MHz bandwidth, R₃₂₀. The smallestallocation of user data for transmission in LTE is a “resource block”comprising twelve sub-carriers transmitted over one slot (0.5sub-frame). Each individual box in the sub-frame grid in FIG. 3Acorresponds to twelve sub-carriers transmitted on one symbol.

FIG. 3 shows in hatching resource allocations for four LTE terminals340, 341, 342, 343. For example, the resource allocation 342 for a firstLTE terminal (UE 1) extends over five blocks of twelve sub-carriers(i.e. 60 sub-carriers), the resource allocation 343 for a second LTEterminal (UE2) extends over six blocks of twelve sub-carriers and so on.

Control channel data is transmitted in a control region 300 (indicatedby dotted-shading in FIG. 3) of the sub-frame comprising the first nsymbols of the sub-frame where n can vary between one and three symbolsfor channel bandwidths of 3 MHz or greater and where n can vary betweentwo and four symbols for channel bandwidths of 1.4 MHz. For the sake ofproviding a concrete example, the following description relates to hostcarriers with a channel bandwidth of 3 MHz or greater so the maximumvalue of n will be 3. The data transmitted in the control region 300includes data transmitted on the physical downlink control channel(PDCCH), the physical control format indicator channel (PCFICH) and thephysical HARQ indicator channel (PHICH).

PDCCH contains control data indicating which sub-carriers on whichsymbols of the sub-frame have been allocated to specific LTE terminals.Thus, the PDCCH data transmitted in the control region 300 of thesub-frame shown in FIG. 3 would indicate that UE1 has been allocated theblock of resources identified by reference numeral 342, that UE2 hasbeen allocated the block of resources identified by reference numeral343, and so on.

PCFICH contains control data indicating the size of the control region(typically between one and three symbols, but four symbols beingcontemplated to support 1.4 MHz channel bandwidth).

PHICH contains HARQ (Hybrid Automatic Request) data indicating whetheror not previously transmitted uplink data has been successfully receivedby the network.

Symbols in the central band 310 of the time-frequency resource grid areused for the transmission of information including the primarysynchronisation signal (PSS), the secondary synchronisation signal (SSS)and the physical broadcast channel (PBCH). This central band 310 istypically 72 sub-carriers wide (corresponding to a transmissionbandwidth of 1.08 MHz). The PSS and SSS are synchronisation signals thatonce detected allow an LTE terminal device to achieve framesynchronisation and determine the cell identity of the enhanced Node Btransmitting the downlink signal. The PBCH carries information about thecell, comprising a master information block (MIB) that includesparameters that LTE terminals use to properly access the cell. Datatransmitted to individual LTE terminals on the physical downlink sharedchannel (PDSCH) can be transmitted in other resource elements of thesub-frame. Further explanation of these channels is provided below.

FIG. 3 also shows a region of PDSCH 344 containing system informationand extending over a bandwidth of R₃₄₄. A conventional LTE frame willalso include reference signals which are discussed further below but notshown in FIG. 3 in the interests of clarity.

The number of sub-carriers in an LTE channel can vary depending on theconfiguration of the transmission network. Typically this variation isfrom 72 sub carriers contained within a 1.4 MHz channel bandwidth to1200 sub-carriers contained within a 20 MHz channel bandwidth (asschematically shown in FIG. 3). As is known in the art, data transmittedon the PDCCH, PCFICH and PHICH is typically distributed on thesub-carriers across the entire bandwidth of the sub-frame to provide forfrequency diversity. Therefore a conventional LTE terminal must be ableto receive the entire channel bandwidth in order to receive and decodethe control region.

As mentioned above, the anticipated widespread deployment of third andfourth generation networks has led to the parallel development of aclass of devices and applications which, rather than taking advantage ofthe high data rates available, instead take advantage of the robustradio interface and increasing ubiquity of the coverage area. Thisparallel class of devices and applications includes MTC devices andso-called machine to machine (M2M) applications, wherein semi-autonomousor autonomous wireless communication devices typically communicate smallamounts of data on a relatively infrequent basis.

Examples of MTC (and M2M) devices include: so-called smart meters which,for example, are located in a customer's house and periodically transmitinformation back to a central MTC server data relating to the customersconsumption of a utility such as gas, water, electricity and so on;“track and trace” applications such as transportation and logisticstracking, road tolling and monitoring systems; remote maintenance andcontrol systems with MTC-enabled sensors, lighting, diagnostics etc.;environment monitoring; point of sales payment systems and vendingmachines; security systems, etc.

Further information on characteristics of MTC-type devices and furtherexamples of the applications to which MTC devices may be applied can befound, for example, in the corresponding standards, such as ETSI TS 122368 V10.530 (2011 July)/3 GPP TS 22.368 version 10.5.0 Release 10) W.

Whilst it can be convenient for a terminal such as an MTC type terminalto take advantage of the wide coverage area provided by a third orfourth generation mobile telecommunication network, there are at presentdisadvantages and challenges to successful deployment. Unlike aconventional third or fourth generation terminal device such as asmartphone, an MTC-type terminal is preferably relatively simple andinexpensive: in addition MTC-devices are often deployed in situationsthat do not afford easy access for direct maintenance orreplacement—reliable and efficient operation can be crucial.Furthermore, while the type of functions performed by the MTC-typeterminal (e.g. collecting and reporting back data) do not requireparticularly complex processing to perform, third and fourth generationmobile telecommunication networks typically employ advanced datamodulation techniques (such as 16 QAM or 64 QAM) on the radio interfacewhich can require more complex and expensive radio transceivers toimplement.

It is usually justified to include such complex transceivers in asmartphone as a smartphone will typically require a powerful processorto perform typical smartphone type functions. However, as indicatedabove, there is now a desire to use relatively inexpensive and lesscomplex devices to communicate using LTE type networks. In parallel withthis drive to provide network accessibility to devices having differentoperational functionality, e.g. reduced bandwidth operation, there is adesire to optimise the use of the available bandwidth in atelecommunications system supporting such devices.

In many scenarios, providing low capability terminals such as those witha conventional high-performance LTE receiver unit capable of receivingand processing (control) data from an LTE downlink frame across the fullcarrier bandwidth can be overly complex for a device which only needs tocommunicate small amounts of data. This may therefore limit thepracticality of a widespread deployment of low capability MTC typedevices in an LTE network. It is preferable instead to provide lowcapability terminals such as MTC devices with a simpler receiver unitwhich is more proportionate with the amount of data likely to betransmitted to the terminal.

A “virtual carrier” tailored to low capability terminals such as MTCdevices is thus provided within the transmission resources of aconventional OFDM type downlink carrier (i.e. a “host carrier”). Unlikedata transmitted on a conventional OFDM type downlink carrier, datatransmitted on the virtual carrier can be received and decoded withoutneeding to process the full bandwidth of the downlink host OFDM carrier,for at least some part of a sub-frame. Accordingly, data transmitted onthe virtual carrier can be received and decoded using a reducedcomplexity receiver unit.

The term “virtual carrier” corresponds in essence to a narrowbandcarrier for MTC-type devices within a host carrier for an OFDM-basedradio access technology (such as WiMAX or LTE).

The virtual carrier concept is described in a number of co-pendingpatent applications (including GB 1101970.0 [2], GB 1101981.7 [3], GB1101966.8 [4], GB 1101983.3 [5], GB 1101853.8 [6], GB 1101982.5 [7], GB1101980.9 [8] and GB 1101972.6 [9]), the contents of which areincorporated herein by reference. For ease of reference, however, anoverview of certain aspects of the concept of virtual carriers is setout in Annex 1.

FIG. 4 schematically represents an arbitrary downlink subframe accordingto the established LTE standards as discussed above into which aninstance of a virtual carrier 406 has been introduced. The subframe isin essence a simplified version of what is represented in FIG. 3. Thus,the subframe comprises a control region 400 supporting the PCFICH, PHICHand PDCCH channels as discussed above and a PDSCH region 402 forcommunicating higher-layer data (for example user-plane data andnon-physical layer control-plane signalling) to respective terminaldevices, as well as system information, again as discussed above. Forthe sake of giving a concrete example, the frequency bandwidth (BW) ofthe carrier with which the subframe is associated is taken to be 20 MHz.Also schematically shown in FIG. 4 by black shading is an example PDSCHdownlink allocation 404. In accordance with the defined standards, andas discussed above, individual terminal devices derive their specificdownlink allocations 404 for a subframe from PDCCH transmitted in thecontrol region 400 of the subframe.

By contrast with the conventional LTE arrangement, where a subset of theavailable PDSCH resources anywhere across the full PDSCH bandwidth couldbe allocated to a UE in any given subframe, in the T-shaped arrangementillustrated in FIG. 4, MTC devices maybe allocated PDSCH resources onlywithin a pre-established restricted frequency band 406 corresponding toa virtual carrier.

Accordingly, MTC devices each need only buffer and process a smallfraction of the total PDSCH resources contained in the subframe toidentify and extract their own data from that subframe.

The pre-established restricted frequency band used to communicate, e.g.on PDSCH in LTE, from a base station to a terminal device, is thusnarrower than the overall system frequency band (carrier bandwidth) usedfor communicating physical-layer control information, e.g. on PDCCH inLTE. As a result, base stations may be configured to allocate downlinkresources for the terminal device on PDSCH only within the restrictedfrequency band. As the terminal device knows in advance that it willonly be allocated PDSCH resources within the restricted frequency band,the terminal device does not need to buffer and process any PDSCHresources from outside the pre-determined restricted frequency band.

In this example it is assumed the base station and the MTC device haveboth pre-established that data is to be communicated from the basestation to the MTC device only within the restricted frequency banddefined by upper and lower frequencies f1 and f2 (having a bandwidthΔf). In this example the restricted frequency band encompasses thecentral part of the overall system (carrier) frequency band BW. For thesake of a concrete example, the restricted frequency band is assumedhere to have a bandwidth (Δf) of 1.4 MHz and to be centred on theoverall system bandwidth (i.e. f1=fc−Δf/2 and f2=fc+Δf/2, where fc isthe central frequency of the system frequency band). There are variousmechanisms by which the frequency band can be established/shared betweena base station and terminal device and some of these are discussedfurther below.

FIG. 4 represents in shading the portions of each subframe for which theMTC device is arranged to buffer resource elements ready for processing.The buffered part of each subframe comprises a control region 400supporting conventional physical-layer control information, such as thePCFICH, PHICH and PDCCH channels as discussed above and a restrictedPDSCH region 406. The physical-layer control regions 400 that arebuffered are in the same resources as the physical-layer control regionsbuffered by any conventional UE. However, the PDSCH regions 406 whichare buffered by the MTC device are smaller than the PDSCH regionsbuffered by conventional UEs. This is possible because, as noted above,the MTC devices are allocated PDSCH resources only within a restrictedfrequency band that occupies a small fraction of the total PDSCHresources contained in the subframe.

Accordingly, the MTC device will in the first instance receive andbuffer the entire control region 400 and the entire restricted frequencyband 406 in a subframe. The MTC device will then process the controlregion 400 to decode PDCCH to determine what resources are allocated onPDSCH within the restricted frequency band, and then process the databuffered during PDSCH symbols within the restricted frequency band andextract the relevant higher-layer data therefrom.

In one example LTE-based implementation, each subframe is taken tocomprise 14 symbols (timeslots) with PDCCH transmitted on the firstthree symbols and PDSCH transmitted on the remaining 11 symbols.Furthermore, the wireless telecommunications system is taken in thisexample to operate over a system frequency band of 20 MHz (100 resourceblocks) with a pre-established restricted frequency band of 1.4 MHz (sixresource blocks) defined for communicating with the terminal devicessupporting virtual carrier operation.

As explained above, in OFDM-based mobile communication systems such asLTE, downlink data is dynamically assigned to be transmitted ondifferent sub-carriers on a sub-frame by sub-frame basis. Accordingly,in every sub-frame, the network signals which sub-carriers on whichsymbols contain data relevant to which terminals (i.e. downlinkallocation signalling).

As can be seen from FIG. 3, in a conventional downlink LTE sub-frameinfo′ nation regarding which symbols contain data relevant to whichterminals is transmitted on the PDCCH during the first symbol or symbolsof the sub-frame.

The concept of virtual carriers provided on blocks of OFDM subcarriersthat are not centred on the host carrier central frequency is known fromco-pending patent application number GB 113801.3 [10], which describesan arrangement where there is a plurality of MTC devices and the centralfrequency of at least some of the virtual carriers is not same as thecentral frequency of the host carrier.

FIG. 5 illustrates this arrangement. A LTE downlink sub-frame is shownwith a plurality of virtual carriers outside of the control region 300,the data region includes a group of resource elements positioned belowthe central band 310 which form a virtual carrier VC3 501. The virtualcarrier VC3 501 is adapted so that data transmitted on the virtualcarrier VC3 501 can be treated as logically distinct from datatransmitted in the remaining parts of the host carrier and can bedecoded without decoding all the control data from the control region300.

FIG. 5 also shows virtual carriers occupying frequency resources abovethe centre band (VC1, 502) and (as in the situation illustrated in FIG.4) including the centre band (VC2, 401).

Therefore, depending on, for example, the expected virtual carrierthroughput, the location of a virtual carrier can be appropriatelychosen to either exist inside or outside the centre band 310 accordingto whether the host or virtual carrier is chosen to bear the overhead ofthe PSS, SSS and PBCH. This band allocation method for multiple VCs hasparticular application when terminals (UEs) using the VC create asignificant quantity of traffic at a given time. It is thereforedesirable that the respective subsets of UEs served by each virtualcarrier can locate control signals relevant to their virtual carrier.

Common and UE Search Spaces for PDCCH

As discussed previously in the context of conventional LTE, at leastsome of the resource elements (REs) comprising a host carrier (FTC)control region are defined by specification to map onto a number ofso-called control channel elements (CCEs). FIG. 6C illustrates thismapping process in more detail. The information bits comprising the CCEsare subjected to a process of cell-specific bit scrambling, QPSKmodulation, an interleaver operating upon groups of the resulting QPSKsymbols, cell-specific shifting of a predetermined number of those QPSKsymbols and then the mapping of those symbols to REs (the dark shadedslots in the left hand region of the subframe structure). Physically,any given CCE is distributed across the REs of the HC control region.

The physical downlink control channel (PDCCH) comprises a number ofCCEs. The number of CCEs comprising a particular PDCCH depends on theaggregation level determined by the eNodeB. A UE must search throughsome number of the CCEs in the control region to determine if there areany that comprise PDCCHs containing control information pertinent to theUE.

The locations of CCEs forming PDCCHs can be arranged by the eNodeB tomake the search process at the UE more efficient by arranging CCEs fordifferent purposes in logical “search spaces”.

Some CCEs are searched (monitored) by all UEs in the cell, these CCEscomprising one or more common search spaces (CSS). The order in whichthe CCEs of the CSSs within each subframe are searched by UEs remainsstatic and is given by the specification (i.e. not configured by RRC).

Some CCEs are not searched by all UEs, these CCEs comprising a pluralityof UE-specific search spaces (UESS). The order in which the CCEs of theUESSs for a given UE within each subframe are searched is dependent uponthe relevant RNTI for that UE: the CCEs with which any one UE beingssearching a UESS will also change between subframes within a radioframe.

A CCE may be part of more than one search space. Typically, PDCCHscomprising CCEs in a common search space contain information relevant toall UEs in a cell and PDCCHs comprising CCEs in a UE-specific searchspace contain information relevant only to one UE.

A typical blind decoding process will make around ten attempts to decodecommon search space. The number of attempts may be restricted as the CSSis limited to only certain DCI formats (i.e. 0, 1A, 3, 3A—see 3 GPP TS36.212)—which carry data relevant to all UEs in the cell. Furthermorethe size of the CSS is restricted to a predefined number of resourceelements (e.g. 144 REs=2 aggregations of 8-CCEs or 4 aggregations of4-CCEs).

By contrast, many more blind decoding attempts (˜30) are typicallyrequired to decode UE-specific search space (UESS) successfully: morepossibilities are available to the eNB in terms of the level ofaggregation applied to UESS (see the discussion of aggregation levelsbelow) and in terms of DCI formats for data directed to specific UEs.

In what follows, unless otherwise indicated or obvious, references to aUE are references to a UE operating on a VC, i.e. a VC-UE.

Group-Based PDCCH Capability

To address the potential capacity problem in the host carrier controlregion (i.e. the PDCCH), exemplary embodiments provide the operation ofa new group-specific search space (GSS) for PDCCH which conveys controlinformation common to a group of the UEs receiving PDCCH in a givensubframe, but which is not common to all such UEs. This group-basedcontrol information may be adapted to inform members of a group ofVC-UEs where a further control channel can be found which containsinformation specific to, on the one hand, the structure and operation ofthe VC and, on the other hand, the usual information conveyed per-UE onPDCCH. More generally this allows information common to a group of UEs,but not intended for broadcast to all UEs, to be signalled efficientlyon PDCCH. By defining group search spaces, more efficient use can bemade of PDCCH capacity without imposing an unnecessary processing loadon non-VC UEs or fundamentally altering the mapping between CCEs and REs(illustrated in FIG. 6C).

Thus a group-based control channel functionality is implemented. Thisfunctionality indicates the location of a further control region, which,in turn, indicates to VC-UEs the behaviour of a VC embedded within a HC.

It should be noted that there may in general be more than one VC inoperation at a time on an HC (depending on scheduling needs, networkconfiguration, etc.)—as illustrated in FIG. 5. Thus there can be morethan one grouped PDCCH in the control region containing information forthe more than one VC.

In certain embodiments, the GSS is identified by CRC scrambling with anew group C-RNTI (G-C-RNTI). One mechanism for assigning the newG-C-RNTI to a UE is to have that identifier assigned by the networkduring the RA procedure.

Assigning Group Identity by Implicit Signalling

Assignment of G-C-RNTIs could be done, for example, by adding anadditional field to the Random Access Response (RAR) to convey theG-C-RNTI, which could be taken from among the reserved RNTI valuesspecified in TS 36.321, or by making reservations among the existingC-RNTIs in specification, or by defining new RNTI values. This approachis not backwards compatible, since legacy UEs would not be able tointerpret the extended RAR this would produce.

The conventional Radio Resource Control (RRC) signalling in the randomaccess procedure is summarised in 3 GPP TS 36.300. The overall(contention based) Random Access (RA) procedure is shown in FIG. 8A.

An RA preamble is sent from a UE to a base station (i.e. an eNodeB). TheUE uses a RA preamble transmission to announce its presence in a celland to allow the eNB to establish the time of flight of the signal fromUE to base station.

The base station constructs a RAR addressed to the RA-RNTI given by theUE. The RA-RNTI is determined by the time and frequency resources inwhich the UE transmitted the RA preamble. The RAR also includes atemporary C-RNTI (a new identifier assigned to the UE while it is in thecell), and an indication of which preamble was received. The structureof the RAR at the MAC layer is described in 3 GPP TS 36.321 andillustrated at FIG. 8B.

The assignment of a group C-RNTI by extending the conventional RAR isillustrated in FIG. 8C.

A method for assigning a group C-RNTI whilst maintaining the currentsize and structure of the RAR, is for the network to construct a RAR asfollows: the RAR is still addressed to RA-RNTI and contains a temporaryC-RNTI (and other information specified in 3 GPP TS 36.321), however theindication of which preamble was received can differ from that which wasactually received (and assumed to have been transmitted by the UE) asdescribed next.

An RA preamble comprises a sequence. There are N=64 such sequences, alsoknown as “RA preamble sequences” or “preamble signatures” or simply“preambles”, defined in a cell. For illustration, consider that thesepreambles are numbered n=0 . . . 63. Normally, the eNB constructs theRAR containing an indication of the same preamble signature numbered n₁as was actually received from (and assumed to have been transmitted by)the UE for which the RAR is intended. However, in this embodiment, theeNB indicates another preamble numbered n₂ in general not equal to n₁such that

${\frac{n_{1}}{n_{2}}{mod}\mspace{14mu} N_{VC}} = g$

where N_(VC) is the number of virtual carrier groups defined in the celland g is the group to which the eNB wishes to assign the UE. The mappingfrom a value of g to a G-C-RNTI may be provided in RRC signalling,specified in a standard, or broadcast.

A UE decoding an RAR addressed to the relevant RA-RNTI therefore obtainsa temporary C-RNTI as usual and infers a G-C-RNTI.

As the UE still selects preamble signature n₁ at random, there is anon-trivial chance that another UE in the cell may select the samepreamble signature—a scenario referred to as “contention”. Should therebe contention on the choice of preamble, conventional methods can beused to resolve it. FIG. 8A shows additional signalling in thespecification of the random access procedure (see 3 GPP TS 36.300)specifically dealing with preamble signature contention.

Another backwards compatible method would be to reserve some temporaryC-RNTI values from the allowed range, or use the reserved range ofRNTIs, and define that a temporary C-RNTI received in the RAR is used asthe G-C-RNTI by compatible UEs. Higher-layer signalling, such as an RRCconfiguration, could indicate whether a compatible UE should actuallypay any regard to a G-C-RNTI acquired in this way.

Yet another example, maintaining the current size and structure of theRAR, is for the network to construct the RAR for a grouped UE to containone of the reserved RNTIs discussed earlier instead of one of the valuesof RNTI permitted for C-RNTI. Normally, a UE would simply ignore such anRAR. However, a UE operating in accordance with this embodimentinterprets an RAR containing a reserved RNTI as signalling that: it canregard its temporary C-RNTI as its permanent C-RNTI; and its G-C-RNTI isthe (reserved) RNTI contained in the RAR.

However the G-C-RNTI is assigned and transmitted to the UE, UEs now inpossession of a G-C-RNTI are expected to search for PDCCHs in the CSSand GSS at least. Note that in certain cases, there is no need for suchUEs to search the UESS on the HC control region for a UE-specific PDCCHsince their UE-specific information may be confined to the VC.Nevertheless, depending on system design and configuration, such UEs mayadditionally search for UE-specific PDCCHs on the HC.

Group-Specific Search Space Operation

PDCCHs in the GSS can use a group DCI (G-DCI). This G-DCI can adopt anexisting DCI format, or use one or more new DCI format(s) which arerestricted specifically to the GSS; the DCI format used being selectedso that the number of blind decodes across DCI formats is limited.Irrespective of format, the G-DCI conveys information relevant to allUEs in the group. Particular examples for the VC include:

-   -   The location of a further control region within the resources of        the VC.    -   The reference signal (RS) structure on the VC, since this may        differ from that in existing specifications and the HC.    -   Carrier aggregation (CA) information specific to aggregated VCs.

The PDCCHs within the VC control region then provide UE-specificinformation regarding the scheduling, etc. on the VC. Note that it istherefore possible that a UE in possession of a G-C-RNTI need not searchfor a UESS on the HC control region (PDCCH), saving a potentiallysignificant amount of blind decoding processing effort.

FIG. 6A shows a LTE downlink radio sub-frame in which a HC controlregion 600 is supplemented by a VC control region 604 (a VC PDCCHregion) within the restricted frequency band of a virtual carrier 606.As in FIG. 4, the regions outside the HC control region 600 and thevirtual carrier region 606 constitute a PDSCH region 602 forcommunicating data (for example user-plane data and non-physical layercontrol-plane signalling) to conventional LTE terminal devices. In thisinstance, the VC control region 604 occupies symbols across the entirerestricted frequency band of the virtual carrier which immediatelysucceed the symbols of the HC control region 600. The VC control regionis not however limited to occupying these particular symbols or indeedto having an extent across the available virtual carrier frequencybands.

Certain REs of the HC control region 600 constitute the CSS and GSS fora VC UE (RE occupation of the search spaces is not shown in detail, andis defined by specification—search spaces are not in contiguous REs ingeneral). A UE searches CSS and GSS, with GSS containing a PDCCHcarrying a DCI indicating the location of the VC control region as wellas other information which is VC-specific for all UEs using theillustrated VC.

Recent developments of the LTE standard have lead to a proposal for theintroduction of a narrow band control channel, Enhanced PhysicalDownlink Control Channel (EPDCCH), supplemental to the PDCCH. The EPDCCHis transmitted over a number of contiguous subcarriers or a number ofsets of contiguous subcarriers, the number of subcarriers in any one setand the total number of subcarriers in all the sets being fewer than thenumber of subcarriers available in a subframe (thus “narrow band”relative to the full bandwidth of the host carrier). By analogy, avirtual carrier may itself implement a narrow band control channel (VCEPDCCH) that extends in a subset of the VC subcarriers over asubstantial part of the subframe.

If the network provides a suitable VC, the VC could include its ownEPDCCH regions as well as PDCCH regions within the resources of the VC.In this case, the GSS on the conventional (wideband) PDCCH can provideadditional control information to the group of UEs regarding how toaccess the VC-EPDCCH. EPDCCH on the HC could be inaccessible since theUE is by assumption narrowband at least outside the HC control region.

FIG. 6B illustrates an example of a LTE downlink radio sub-frame inwhich the HC control region 600 is supplemented by a VC control region604 (a VC PDCCH region) and a VC-EPDCCH region 608 within the restrictedfrequency band of a virtual carrier 606, together with an EPDCCH controlregion 610. The regions outside the HC control region 600, the virtualcarrier region 606 and the EPDCCH control region 610 constitute a PDSCHregion 602 for communicating data (for example user-plane data andnon-physical layer control-plane signalling) to conventional LTEterminal devices. As in FIG. 6A, the VC control region 604 occupiessymbols across the entire restricted frequency band of the virtualcarrier which immediately succeed the symbols of the HC control region600. The VC-EPDCCH region 608 occupies a region of the subframe, withinthe restricted frequency band of a virtual carrier 606, which extendsacross symbols in a subset of the virtual carrier subcarriers which aredistinct from the symbols of the HC and VC control regions. The EPDCCHcontrol region 610 occupies symbols in a different subset of subcarriersdistinct from the symbols of the HC control region 600.

As for FIG. 6A, certain REs of the HC control region 600 constitute theCSS and GSS for a VC UE. A UE decodes CSS and GSS, with GSS containing agroup-specific indication of the location of the ‘legacy’ VC controlregion 604 (corresponding to the PDCCH) and the VC EPDCCH 608, alongwith other VC-specific information.

In conventional solutions, access to VC-EPDCCH 608 could require furthersignalling on the legacy control region 604 (VC-PDCCH). However, giventhe restricted bandwidth and desire to minimise the overhead in theserestricted resources, having to signal to VC-UEs the location of VCEPDCCH 608 from within the VC could be overhead-heavy. Instead, incertain embodiments, the GSS on the HC's PDCCH 600 can be used toprovide all relevant UEs with direct access to the VC-EPDCCH 608 (aswell as VC-PDCCH 604, if desired), freeing up resource on the VC and inits control region.

Location of GSS

As noted above, the location (start point) of each UE's UESS among thevarious CCEs can change per subframe to reduce the possibility ofscheduling conflicts making it impossible to schedule all desired UEsfor successive subframes. The CSS on the other hand is fixed in locationto reduce the search load for UEs. Since there could be more than onegroup-based PDCCH, at least one per VC, the same location (start point)nature for the GSS could be appropriate as for the UESS, i.e. thatlocation should ideally change on a per subframe basis. The location(start point) of the GSS could be determined based on the G-C-RNTIassigned to the group, in a similar manner to the start point ofsearching UESS being defined by the conventional C-RNTI assigned to aUE.

Power Control in PDCCH

There is no support for power control for PDCCH. Instead of powercontrol, SINR adjustment based on the number of Control Channel Elements(CCE) is applied. A CCE corresponds to nine resource element (RE) groups(also known as “quadruplets”). A physical control channel (i.e. PDCCH)is transmitted on one CCE or an aggregation of several consecutive CCEs:the LTE standard has “aggregations” of one, two, four and eight CCEs.The number of CCEs in an aggregation is referred to as the “aggregationlevel”. While power is not directly controlled, the aggregation of CCEscan increase the total power expended transmitting the PDCCH, therebyincreasing the effective range from the physical location of an eNBwithin which it may be received by UEs.

FIG. 6C illustrates the correspondence between CCEs and resource element(RE) groups. In general, the CCEs map to REs that are distributed acrossthe n OFDM symbols of the HC control region. Aggregating more CCEs totransmit a PDCCH means that a greater proportion of the REs in the HCcontrol region are devoted to that PDCCH.

As noted previously, in terms of blind decoding, the permittedaggregation level is one parameter that can be used to restrict thenumber of attempts at decoding the PDCCH data.

Determining Aggregation Level of GSS

The Aggregation Level Appropriate to Transmission of a PDCCH MayTypically be determined by the size of the DCI it carries and the totalpower with which the UE needs to receive it, according to e.g. radiochannel conditions. To reduce blind search load, the PDCCHs in the CSSare transmitted at an aggregation level only of 4 or 8, while the UESSuses aggregation levels of 1, 2, 4 or 8. The PDCCHs in the GSS, sincethey could apply to a UE at any distance from the eNB, anywhere in thecell, may be best suited to using aggregation levels 4 or 8, like theCSS.

Since a UE must already search two search spaces, it is desirable toreduce the processing load from the introduction of the GSS. Somemethods for implementing the GSS are now described which achieve thisaim.

In a first case, the aggregation level of PDCCHs in the GSS could betransmitted on PBCH (the physical broadcast channel—in the firstsubframe of each radio frame) by using some of the unused bits of themaster information block (MIB). This allows updates to the aggregationlevel once per radio frame when a new MIB can be transmitted.

In a second case, the aggregation level of the PDCCHs in the GSS islinked to that of the CSS. As each PDCCH found in the CSS could be ataggregation level 4 or 8 independently, the linkage may be to anaggregation level determined from one or more of those of the PDCCHfound in the CSS. For example, the aggregation level of PDCCHs in theGSS could be defined in specifications to be the same as that of thehighest aggregation level used on CSS or some other fixed function ofthat of the PDCCHs in the CSS, leaving the eNB free to set theaggregation of PDCCHs in the CSS. Further, the UE may be expected todecode the PDCCHs from the CSS before attempting the GSS. Thus, forexample if a CSS aggregation level is 8, the GSS could have aggregationlevel 4 or it could simply be 8, as for the CSS.

In a third case, the aggregation level of PDCCHs in the GSS is apredetermined function of the G-C-RNTI. However, this means that theaggregation level used by a UE can only change when its G-C-RNTIchanges, which may be only slowly when the group it identifies isdisbanded, for example.

In a fourth case, the aggregation level to be used in transmission maybe determined by the eNodeB, that determination being dependent onfeedback, or lack of expected feedback, from UE/UEs. It is noted thatUEs successfully decoding the grouped PDCCH in a GSS are expected thento decode at least VC-PDCCH and VC-PDSCH if appropriate. At least oneamong the UEs in a group will then be expected to eventually sendfeedback, i.e. as HARQ ACK/NACKs on PUCCH or PUSCH. However, if the GSSis not successfully searched or decoding of the group PDCCH fails atsome UEs, some UEs will send neither ACK nor NACK and so the rate ofreceiving ACK/NACKs on PUCCH or PUSCH will be lower than expected. Thiswould imply that the aggregation level of the grouped PDCCH may not behigh enough to ensure good decoding success probability and the eNBcould then increase the aggregation level as appropriate, e.g. from 4 to8. If the rate of ACK/NACK reception is at an expected level, the eNBcould try reducing the aggregation level of the group PDCCH, e.g. from 8to 4, until the rate of ACK/NACK reception falls below a suitable level.

Of course, the last technique may be used in conjunction with any one ofthe previous cases: an initial or default aggregation level being set byone of the first three schemes and then dynamically updated dependingupon decoding success probability.

As seen in the preceding sections, the GSS would preferably beimplemented with a mixture of the characteristics of the CSS(aggregation level) and UESS (location) reflecting its distinctivepurpose.

As noted above, there is the possibility that a UE will fail tosuccessfully decode its group-based PDCCH, or incorrectly decode agrouped PDCCH not intended for it. In these cases, that UE will beunable to access the remainder of the subframe. However, this behaviouris the same as that in the case of failure on CSS or UESS. Notably,because each UE is responsible only for decoding its own grouped PDCCH,a failure of one UE does not necessarily imply failure of another UE.

As noted above embodiments of the invention may in particular beemployed within the context of what might be termed “virtual carriers”operating within a bandwidth of one or more “host carriers”. Theconcepts of virtual carriers are described in Annex 1.

FIG. 9 provides a schematic diagram showing part of an adapted LTEmobile telecommunication system arranged in accordance with anembodiment of the present invention. The system includes an adaptedenhanced Node B (eNB) 1401 connected to a core network 1408 whichcommunicates data to a plurality of conventional LTE terminals 1402 andreduced capability terminals 1403 within a coverage area (cell) 1404.Each of the reduced capability terminals 1403 has a transceiver unit1405 which includes a receiver unit capable of receiving data across areduced bandwidth (i.e. narrowband) and a transmitter unit capable oftransmitting data across a reduced bandwidth when compared with thecapabilities of the transceiver units 1406 included in the conventionalLTE terminals 1402.

The adapted eNB 1401 is arranged to transmit downlink data using asub-frame structure that includes a virtual carrier as described withreference to FIG. 11. The task of assigning reduced capacity terminals1403 to a given virtual carrier is performed by a radio resourcemanagement (RRM) unit 1411 within the eNB 1401. Data is then transmittedto reduced capability terminals 1403 by an adapted scheduling unit 1409in the eNB. The reduced capability terminals 1403 are thus able toreceive and transmit data using the downlink virtual carriers asdescribed above.

As has been explained above, because the reduced complexity terminals1403 receive and transmit data across a reduced bandwidth on the uplinkand downlink virtual carriers, the complexity, power consumption andcost of the transceiver unit 1405 needed to receive and decode downlinkdata and to encode and transmit uplink data is reduced compared to thetransceiver unit 1406 provided in the conventional LTE terminals.

When receiving downlink data from the core network 1408 to betransmitted to one of the terminals within the cell 1404, the adaptedeNB 1401 is arranged to queue that data in a queue 1410 and to determineif the data is bound for a conventional LTE terminal 1402 or a reducedcapability terminal 1403. This can be achieved using any suitabletechnique. For example, data bound for a reduced capability terminal1403 may include a virtual carrier flag indicating that the data must betransmitted on the downlink virtual carrier. If the adapted eNB 1401detects that downlink data is to be transmitted to a reduced capabilityterminal 1403, an adapted scheduling unit 1409 included in the adaptedeNB 1401 ensures that the downlink data is transmitted to the reducedcapability terminal in question on the downlink virtual carrier. Inanother example the network is arranged so that the virtual carrier islogically independent of the eNB. More particularly the virtual carriermay be arranged to appear to the core network as a distinct cell so thatit is not known to the core network that the virtual carrier has anyrelationship with the host carrier. Packets are simply routed to/fromthe virtual carrier just as they would be for a conventional cell.

In another example, packet inspection is performed at a suitable pointwithin the network to route traffic to or from the appropriate carrier(i.e. the host carrier or the virtual carrier).

In yet another example, data from the core network to the eNB iscommunicated on a specific logical connection for a specific terminaldevice. The eNB is provided with information indicating which logicalconnection is associated with which terminal device. Information is alsoprovided at the eNB indicating which terminal devices are virtualcarrier terminals and which are conventional LTE terminals. Thisinformation could be derived from the fact that a virtual carrierterminal would initially have connected using virtual carrier resources.

Virtual carrier terminals are arranged to indicate their capability tothe eNB during the connection procedure. Accordingly the eNB can mapdata from the core network to a specific terminal device based onwhether the terminal device is a virtual carrier terminal or an LTEterminal.

In some examples, the virtual carrier inserted within the host carriercan be used to provide a logically distinct “network within a network”.In other words data being transmitted via the virtual carrier can betreated as logically and physically distinct from the data transmittedby the host carrier network. The virtual carrier can therefore be usedto implement a so-called dedicated messaging network (DMN) which is“laid over” a conventional network and used to communicate messagingdata to DMN devices (i.e. virtual carrier terminals).

It will be appreciated that various modifications can be made to theembodiments described above without departing from the scope of thepresent invention as defined in the appended claims. In particularalthough embodiments of the invention have been described with referenceto an LTE mobile radio network, it will be appreciated that the presentinvention can be applied to other forms of network such as GSM, 3G/UMTS,CDMA2000, etc. The term MTC terminal as used herein can be replaced withuser equipment (UE), mobile communications device, terminal device etc.Furthermore, the term base station refers to any wireless network entitythat provides UEs with an air interface to a cellular telecommunicationsnetwork: while the term has been used interchangeably with e-NodeB inthe foregoing it should be understood that it encompasses equivalentnetwork entities in LTE and alternative radio access architecturesincluding: eNode-Bs; Node-Bs, pico-, femto- and micro base stationequipment, relays; boosters etc.

The implementations of the invention described herein may requirealterations in the operation of the UEs themselves. A key difference inUE behaviour compared to conventional operation is that the UE firstsearches the CSS on PDCCH and then must search at least the GSS beforebeing able to decode a further control region embedded in a virtualcarrier within a host carrier, only after which can it proceed to accessUE-specific information regarding, among other things, the resources forDL/UL transmission in the rest of the subframe.

FIG. 10 illustrates these steps. In step 1002, data is received from thePDCCH in the HC control region. The GSS is located (step 1004) withinthe HC control region using a shared group identifier. The GSS PDCCH isprocessed to extract group-specific information (step 1006) and toobtain information indicative of the location of the further controlregion (a VC control region, say) (step 1008). Using the location of thefurther control region, data is received from the further control region(step 1010) and further group-specific information may optionally alsobe extracted (step 1012; shown in dashed lines). Any CSS on the VC-PDCCHis effectively group-specific information as it is extracted by virtueof access to a control region specific to the group.

In conventional systems, the HC control region is searched for commonand UE-specific information, and then the UE can proceed as normal inthe remainder of the subframe.

A further difference lies in the UE and network behaviour for obtainingthe G-C-RNTI. In at least one backwards compatible embodiment, the UEmay be required to determine whether its temporary C-RNTI has beenconfirmed indirectly by reference to another RNTI provided in the RAR,whereas the C-RNTI is provided directly without a further step inconventional operation.

A further difference relates to obtaining the G-C-RNTI. The G-C-RNTI inthe backwards compatible example is confirmed indirectly as a functionof another RNTI value rather than by being provided directly.

As discussed above, in certain embodiments, a new compact downlinkcontrol information (DCI) format is specified, this new DCI format isused only in the GSS, relieving relevant UEs of a significant degree ofblind decoding.

It is further noted that using the GSS to indicate the location of theVC-PDCCH could remove the need for a PCFICH-like functionality to beimplemented on the VC (i.e. a separate channel for indicating the sizeof the VC control region may be unnecessary), saving valuable resource.

By establishing a GSS in the manner described above, the PDCCH can nowprovide control indication to a subset of all UEs, i.e. controlinformation may now be “multicast” to a limited number of UEs ratherthan broadcast or UE-specific.

There is no provision in current specifications for an RNTI to be sharedamong a subset of all UEs: RNTIs other than C-RNTI are cell-specific.

TPC-PUCCH-RNTI and TPC-PUSCH-RNTI are group-based UE identifiers forPDCCHs. They are identifiers specific to power control procedures andsignalled in certain DCI formats on the CSS. They are not howeverassociated with a search space in their own right, being relevant onlyto the CSS, and only part of the message carried by the power controlcommand is relevant to a particular UE, identified by RRC in theTPC-PDCCH-Config IE. The differences are illustrated in FIG. 7.

PDCCH does not at present include information regarding the structure ofa VC, nor the elements of the construction of a carrier identified inthe description above.

Annex 1

The virtual carrier concept is described in a number of co-pending UKpatent applications (including GB 1101970.0 [2], GB 1101981.7 [3], GB1101966.8 [4], GB 1101983.3 [5], GB 1101853.8 [6], GB 1101982.5 [7], GB1101980.9 [8] and GB 1101972.6 [9]). Certain aspects of the concept ofvirtual carriers are set out below. In this section, the followingabbreviations are frequently adopted: virtual carrier—VC, hostcarrier—HC, user equipment—UE, resource block—RB, radio frequency—RF,and baseband—BB.

Like conventional OFDM, the virtual carrier concept has a plurality ofsubcarriers disposed at predetermined offsets from a central frequency:the central frequency thus characterises the entire virtual carrier.

Typical virtual carrier bandwidth is six resource blocks, (i.e. 72sub-carriers) which is in line with minimum 3 GPP bandwidth in LTE.However, as will be seen in the following description, the bandwidth ofVC is by no means restricted to 6 RBs.

In line with Release 8 of the 3 GPP standard for LTE (REL8 LTE), VCresources are typically located in the resource blocks centred on thehost carrier centre frequency and symmetrically allocated (at eitherside of that HC centre frequency) regardless of system bandwidth.

FIG. 4 is a schematic diagram of a grid which illustrates the structureof a downlink LTE sub-frame with a virtual carrier 406 occupying theresource blocks centred on the host carrier centre frequency. Thevirtual carrier central frequency, (f2+f1)/2, is selected to be thecentral frequency, fc, of the host carrier.

In keeping with a conventional LTE downlink sub-frame illustrated inFIG. 3, the first n symbols form the control region 400 which isreserved for the transmission of downlink control data such as datatransmitted on the PDCCH, PCFICH or PHICH.

The signals on the virtual carrier 406 are arranged such that signalstransmitted by the host carrier that a terminal device operating on thehost carrier would require for correct operation and expect to find in aknown pre-determined location (e.g. the PSS, SSS, and PBCH in thecentral band 310 in FIG. 3) are maintained.

Before a conventional LTE terminal can begin transmitting and receivingdata in a cell, it first camps on to the cell. Similarly, an adaptedcamp-on process can be provided for terminals using the virtual carrier.A suitable camp-on process for virtual carriers is described in detailin GB 1113801.3 [10]: this camp-on process is incorporated herein byreference.

As described in GB 1113801.3 [10], both “conventional LTE” and virtualcarrier implementations could conveniently include location informationfor the virtual carrier within the PBCH, which already carries theMaster Information Block (MIB) in the host carrier centre band.Alternatively, virtual carrier location information could be provided inthe centre band, but outside of the PBCH. It can for example be alwaysprovided after and adjacent to the PBCH. By providing the locationinformation in the centre band but outside of the PBCH, the conventionalPBCH is not modified for the purpose of using virtual carriers, but avirtual carrier terminal can easily find the location information inorder to detect the virtual carrier, if any.

In the T-shaped operation of FIG. 4, the virtual carrier locationinformation, if provided, can be provided elsewhere in the host carrier.In other implementations of virtual carriers it may be advantageous toprovide this information in the centre band, for example because avirtual carrier terminal may configure its receiver to operate in anarrow band about the centre band and the virtual carrier terminal thendoes not need to adjust its receiver settings for finding the locationinformation.

Depending on the amount of virtual carrier location informationprovided, the virtual carrier terminal can either adjust its receiver toreceive the virtual carrier transmissions, or it may require furtherlocation information before it can do so.

If for example, the virtual carrier terminal was provided with locationinformation indicating a virtual carrier presence and/or a virtualcarrier bandwidth but not indicating any details as to the exact virtualcarrier frequency range, or if the virtual carrier terminal was notprovided with any location information, the virtual carrier terminalcould then scan the host carrier for a virtual carrier (e.g. performinga so-called blind search process). This process too is discussed indetail in GB 1113801.3 [10].

The reader will readily appreciate that multiple instances of virtualchannels may be implemented at different frequency ranges within thesame cell. FIG. 5 shows a schematic diagram of a downlink LTE subframeexhibiting three different virtual channels.

The present application claims the Paris convention priority ofGB1221729.5 and GB 1221717.0 the contents of which are incorporatedherein by reference.

The following numbered clauses provide further example aspects andfeatures of the present technique:

1. A mobile terminal comprising a processor and a receiver configured toreceive data from a wireless communications system via a wireless accessinterface, the mobile terminal belonging to a predetermined group ofmobile terminals, the wireless access interface providing a plurality ofcommunications resource elements across a first frequency range,

wherein the wireless access interface includes a plurality of timedivided sub-frames, and at least one of the sub-frames includes:

a first control region in a first part of the sub-frame forcommunicating first signalling information to one or more of the mobileterminals, the first control region including a plurality of controlchannel resource elements, a first subset of said control channelresource elements providing a group control channel, the group controlchannel being associated with a group identifier and at least partiallyencoded using the group identifier; and

a second control region in a second part of the sub-frame, distinct fromthe first part of the sub-frame, the second control region being forcommunicating second signalling information to the predetermined groupof the mobile terminals,

and wherein the group control channel contains information indicative ofthe location of the second control region, said information beingaccessible by applying the group identifier to the group controlchannel;

wherein membership of the predetermined group is indicated by the use ofa shared group identifier, and

wherein the processor operates to locate the group control channelwithin the first control region using the shared group identifier.

2. A mobile terminal according to clause 1, wherein the first part ofthe subframe is transmitted before the second part.

3. A mobile terminal according to clause 1 or 2, wherein the groupidentifier is a RNTI and the encoding of the group control channel isthe encoding of CRC bits using the RNTI.

4. A mobile terminal according to any of clauses 1, 2 or 3, wherein thegroup identifier is determined to be a temporary RNTI assigned to thecommunications device by a base station.

5. A mobile terminal according to clause 3, wherein the group identifieris deduced from a relationship between a first preamble signatureindicated in a random access preamble message received by a base stationfrom a terminal and a second preamble signature indicated in a randomaccess response message transmitted by the base station to the terminalin reply to the random access preamble message.

6. A mobile terminal according to clause 5, wherein the random accessresponse message transmitted by the base station in reply to the randomaccess preamble message includes a reserved RNTI where a cell RNTI isexpected, and wherein the group identifier is determined by the terminalto be the reserved RNTI by virtue of the presence of the reserved RNTIin the random access response message.

7. A mobile terminal according to any of clauses 1 to 6, wherein thesecond control region is a narrow band control region, having a secondbandwidth which is less than the bandwidth of the first control region.

8. A mobile terminal according to clause 7, wherein said second controlregion is a region within a third region, the third region beingdistinct from the first control region, the third region having a thirdbandwidth which is less than the bandwidth of the first control regionand greater than or equal to the second bandwidth, the third regionbeing configured for communicating data to one or more of the mobileterminals.

9. A mobile terminal according to clause 8, wherein said third regionincludes a fourth region, the fourth region having a fourth bandwidthwhich is less than the third bandwidth, the fourth region beingconfigured for communicating further control data to one or more of themobile terminals.

10. A mobile terminal according to clause 9, wherein said fourth regionextends across substantially all of the duration of the second part ofthe subframe external to the second control region.

11. A mobile terminal according to any of clauses 1 to 10,

wherein the control channel resource elements of the group controlchannel have an associated aggregation level,

wherein a second subset of said control channel resource elements in thefirst control region provide at least one further control channel, thecontrol channel resource elements of the or each further control channelhaving an associated further aggregation level, and

wherein said aggregation level is a function of at least one of thefurther aggregation levels.

12. A mobile terminal according to any of clauses 1 to 11,

wherein the control channel resource elements of the group controlchannel have an associated aggregation level, and

wherein the aggregation level is broadcast.

13. A mobile terminal according to any of clauses 1 to 12,

wherein the control channel resource elements of the group controlchannel have an associated aggregation level,

wherein the aggregation level is determined by the base stationaccording to feedback information from the mobile terminals, and

wherein the feedback information corresponds to acknowledgement ofreceipt.

14. A mobile terminal according to clause 13,

wherein the feedback information is the result of a comparison betweenan expected rate of receipt of acknowledgements and a detected rate ofreceipt of acknowledgements.

REFERENCES

-   -   [1] ETSI TS 122 368 V10.530 (2011 July)/3 GPP TS 22.368 version        10.5.0 Release 10)    -   [2] UK patent application GB 1101970.0    -   [3] UK patent application GB 1101981.7    -   [4] UK patent application GB 1101966.8    -   [5] UK patent application GB 1101983.3    -   [6] UK patent application GB 1101853.8    -   [7] UK patent application GB 1101982.5    -   [8] UK patent application GB 1101980.9    -   [9] UK patent application GB 1101972.6    -   [10] UK patent application GB 1113801.3    -   [11] UK patent application GB 1121767.6

The invention claimed is:
 1. A mobile terminal comprising a processorand a receiver configured to receive data from a wireless communicationssystem via a wireless access interface, the mobile terminal belonging toa predetermined group of mobile terminals, the wireless access interfaceproviding a plurality of communications resource elements across a firstfrequency range, wherein the wireless access interface includes aplurality of time divided sub-frames, and at least one of the sub-framesincludes: a first control region in a first part of the sub-frame forcommunicating first signaling information to one or more of the mobileterminals, the first control region including a plurality of controlchannel resource elements, a first subset of said control channelresource elements providing a group control channel, the group controlchannel being associated with a group identifier and at least partiallyencoded using the group identifier; and a second control region in asecond part of the sub-frame, distinct from the first part of thesub-frame, the second control region being for communicating secondsignaling information to the predetermined group of the mobileterminals, and the group control channel contains information indicativeof the location of the second control region, said information beingaccessible by applying the group identifier to the group controlchannel, membership of the predetermined group is indicated by the useof the group identifier, the processor operates to locate the groupcontrol channel within the first control region using the groupidentifier; and the group identifier is deduced from a relationshipbetween a first preamble signature indicated in a random access preamblemessage received by a base station from a terminal and a second preamblesignature indicated in a random access response message transmitted bythe base station to the terminal in reply to the random access preamblemessage.
 2. The mobile terminal as claimed in claim 1, wherein the firstpart of the subframe is transmitted before the second part.
 3. Themobile terminal as claimed in claim 1, wherein the group identifier is aRNTI and the encoding of the group control channel is the encoding ofCRC bits using the RNTI.
 4. The mobile terminal as claimed in claim 1,wherein the group identifier is determined to be a temporary RNTIassigned to the communications device by a base station.
 5. The mobileterminal as claimed in claim 1, wherein the random access responsemessage transmitted by the base station in reply to the random accesspreamble message includes a reserved RNTI where a cell RNTI is expected,and wherein the group identifier is determined by the terminal to be thereserved RNTI by virtue of the presence of the reserved RNTI in therandom access response message.
 6. The mobile terminal as claimed inclaim 1, wherein the second control region is a narrow band controlregion, having a second bandwidth which is less than the bandwidth ofthe first control region.
 7. The mobile terminal as claimed in claim 6,wherein said second control region is a region within a third region,the third region being distinct from the first control region, the thirdregion having a third bandwidth which is less than the bandwidth of thefirst control region and greater than or equal to the second bandwidth,the third region being configured for communicating data to one or moreof the mobile terminals.
 8. The mobile terminal as claimed in claim 7,wherein said third region includes a fourth region, the fourth regionhaving a fourth bandwidth which is less than the third bandwidth, thefourth region being configured for communicating further control data toone or more of the mobile terminals.
 9. The mobile terminal as claimedin claim 8, wherein said fourth region extends across the second part ofthe subframe external to the second control region.
 10. The mobileterminal as claimed in claim 1, wherein the control channel resourceelements of the group control channel have an associated aggregationlevel, a second subset of said control channel resource elements in thefirst control region provide at least one further control channel, thecontrol channel resource elements of the or each further control channelhaving an associated further aggregation level, and said aggregationlevel is a function of at least one of the further aggregation levels.11. The mobile terminal as claimed in claim 1, wherein the controlchannel resource elements of the group control channel have anassociated aggregation level, and the aggregation level is broadcast.12. The mobile terminal as claimed in claim 1, wherein the controlchannel resource elements of the group control channel have anassociated aggregation level, the aggregation level is determined by thebase station according to feedback information from the mobileterminals, and the feedback information corresponds to acknowledgementof receipt.
 13. The mobile terminal as claimed in claim 12, wherein thefeedback information is the result of a comparison between an expectedrate of receipt of acknowledgements and a detected rate of receipt ofacknowledgements.
 14. A method performed by a mobile terminal comprisinga processor and a receiver configured to receive data from a wirelesscommunications system via a wireless access interface, the mobileterminal belonging to a predetermined group of mobile terminals, thewireless access interface providing a plurality of communicationsresource elements across a first frequency range, the method comprising:receiving, by the receiver via the wireless access interface, aplurality of time divided sub-frames, at least one of the sub-framesincluding: a first control region in a first part of the sub-frame forcommunicating first signaling information to one or more of the mobileterminals, the first control region including a plurality of controlchannel resource elements, a first subset of said control channelresource elements providing a group control channel, the group controlchannel being associated with a group identifier and at least partiallyencoded using the group identifier; and a second control region in asecond part of the sub-frame, distinct from the first part of thesub-frame, the second control region being for communicating secondsignaling information to the predetermined group of the mobileterminals, wherein the group control channel contains informationindicative of the location of the second control region, saidinformation being accessible by applying the group identifier to thegroup control channel, and membership of the predetermined group isindicated by the use of the group identifier; and locating the groupcontrol channel within the first control region using the groupidentifier, wherein the group identifier is deduced from a relationshipbetween a first preamble signature indicated in a random access preamblemessage received by a base station from a terminal and a second preamblesignature indicated in a random access response message transmitted bythe base station to the terminal in reply to the random access preamblemessage.